Scanning microtelescopes such as arthrascopes and cystoscopes are frequently used to examine the interior of the body by threading an optical observation head through a narrow passage in the body. In order to minimize the discomfort to the patient and permit access to a maximum number of body areas the optical observation head must be extremely small, with a diameter of as little as 3 mm. It is obviously very difficult to place any movable objective lens system in such a small enclosure, and even more difficult to actuate it through the long flexible catheter which connects the head to the eyepiece on the outside of the body.
Because of the size constraints and the need for the entire microtelescope to be sterilizable, known expedients such as movable mirrors cannot be used in surgical microtelescopes. Consequently, it has previously been necessary to provide separate instruments for each offset angle (i.e. the angle between the axis of the head and the axis of the field of vision being examined. Thus, if a surgeon wished to observe a body cavity at a 30.degree. viewing angle and then at a 70.degree. viewing angle, the first instrument had to be withdrawn and a second instrument inserted, thereby increasing patient discomfort and the risk of injury.